Semiconductor wafer processing systems generally contain a process chamber having a pedestal or susceptor for supporting a semiconductor wafer within the chamber proximate a processing region. The chamber forms a vacuum enclosure defining, in part, the process region. A gas distribution assembly or showerhead provides one or more process gases to the process region. The gases may then be heated and/or supplied with energy to form a plasma which performs certain processes upon the wafer. These processes may include chemical vapor deposition (CVD) to deposit a film upon the wafer, or an etch reaction to remove material from the wafer.
As the size and complexity of semiconductor devices has increased, wafer real estate has become more valuable. Consequently, it is desirable to locate devices not only near the center of the wafer, but as close to the outer edge of the wafer as possible. Location of devices near the wafer periphery has increased the demands on the radial uniformity of wafer processing steps. As a result, it is desirable if semiconductor fabrication processes achieve uniformity across nearly the entire wafer surface.
FIG. 2 shows a prior art deposition chamber 210 with a prior art showerhead 220. The prior art showerhead 220 features a plurality of equally spaced holes 222 in the lower surface 225 of the showerhead. Process gases flow into the showerhead 220 through the inlet pipe 214 along the direction marked 215. The holes 222 serve to distribute the process gases along directions 218 inside the showerhead. The process gases exit the showerhead through holes 222 and interact with the surface of the semiconductor wafer 230. The spatial distribution of the gases inside the showerhead determines the uniformity of gas distributed across the surface of the semiconductor wafer.
During a deposition process, the process gases flow over the top surface 235 of the semiconductor wafer 230 and react with the surface 235 or with other gaseous species to form the desired film 236 on the wafer surface 235. The gases flow in directions 238 over the edge of the wafer and are exhausted through the annular exhaust port 250.
In the prior art deposition chamber illustrated in FIG. 2, to reach the exhaust port 250, process gases introduced by the showerhead over the center of the wafer generally flow in a radial direction along the wafer surface and over the edges of the wafer along directions 238. Therefore, the velocity of gaseous species may increase as the gases flow in a radial direction toward the edge of the wafer.
In a deposition process, the rate of deposition typically depends on the flow of reactive species to the semiconductor wafer surface. If the velocity of reactive species increases in the radial direction, the deposition rate may be greater near the wafer periphery than near the wafer center, resulting in non-uniform film thickness.
Therefore, there is a need in the art for an apparatus exhibiting improved uniformity of films deposited on semiconductor wafers.